This invention relates to a high-strength, coated cemented carbide product and to a process for its preparation.
Cemented carbides are well known for their unique combination of hardness, strength and wear resistance and are accordingly extensively used for such industrial applications as cutting tools, drawing dies and wear parts. It is known that the wear resistance of cemented carbides may be enhanced by the application of a thin coating of a highly wear-resistant material, such as, for example, titanium carbide, and such coated cemented carbides are finding increasing commercial utility for certain cutting tool and machining applications. However, the increased wear resistance of such coated products has been at the sacrifice of the strength of the substrate which is substantially reduced after coating.
Because of its high hardness, wear resistance and low reactivity with a wide variety of metals, aluminum oxide has excellent potential as a tool material, and this potential has to some extent been realized with a variety of aluminum oxide cutting materials that are commercially available. The principal drawback to the more widespread use of aluminum oxide tools in their low strength which rarely exceeds 100,000 p.s.i., using the standard transverse rupture or bend test. This compares with a strength of from 200,000 to 300,000, or even more, for cemented carbide cutting tools. The low strength of aluminum oxide tools limits their use to cutting applications where the tool is not highly stressed, such as in finishing cuts. The low strength of aluminum oxide also precludes the use of such materials in certain types of insert shapes which encounter high stresses when locked in a toolholder.
It is an object of this invention to provide a hard, wear-resistant material which combines the extremely high wear resistance of aluminum oxide with the relatively high strength and hardness of cemented carbide.
It is an additional object of this invention to improve the wear resistance of cemented carbides without substantially reducing their strength. It is still an additional object of this invention to provide a process for producing a firmly adherent, nonporous, dense coating of aluminum oxide on a cemented carbide substrate.